By far, the most common means of supplying heat for industrial use is by gas burners. The great majority of these burners are nozzle-mixing burners; that is, fuel gas and combustion air are piped separately to the burner and mixed, for combustion, at the burner nozzle or nozzles. The other type of burner is commonly called a pre-mix burner; combustion air and fuel gas are mixed prior to reaching this type of burner and are ignited at the burner discharge. This invention relates to nozzle-mixing burners.
Industrial burners may be used to heat furnaces, ovens, or other devices or process. In this specification, the word furnace will be used to include all furnaces, ovens, other devices and processes.
Generally, combustion air is supplied to the furnace burners by means of an electrically-driven combustion air blower. At most installations, fuel gas is supplied at an adequate pressure, so that no fuel gas pump or blower is required. In many cases, a multiplicity of burners are connected by common combustion air and fuel gas lines. The most common means of controlling the firing rate of the burner or burners is by controlling the flow of combustion air to the burner or burners, in response to the output of a temperature controller. Varying combustion air flow rates result in varying combustion air static pressures. In order to maintain a pre-determined air/fuel ratio, the fuel gas flow rate is held proportional to the combustion air flow rate, usually by using a regulator in the fuel gas line to maintain the fuel gas static pressure equal to, or proportional to, the combustion air static pressure.
Many other means are used to control air/fuel ratios to burners, including electronic mass flow metering and control systems.
Regardless of the type of burner control system, a common problem encountered in industrial heating applications, where modulated burner input is desired, is the inability, with existing burners, to reduce the total input to the furnace far enough while maintaining the desired air/fuel ratio.
In the industrial heating industry, turndown is the term used to indicate the ratio of the maximum firing rate capability of a burner to its minimum firing rate capability, while maintaining a constant air to fuel ratio. Typically, a nozzle-mixing burner is rated as capable of achieving a turndown of 10 to 1; that is, its minimum firing rate is one-tenth of its maximum firing rate.
In actual practice, turndown is often limited to about 7 to 1 because of the difficulty in controlling the low combustion air and fuel gas static pressures required for very low inputs, particularly the fuel gas static pressure. Where several burners are manifolded together, the very low static pressure result in problems in distributing, evenly, combustion air and fuel gas to the burners comprising the manifold.
For example, if the combustion air static pressure at maximum firing rate were, say, 16 ounces per square inch, the static pressure would be 0.16 ounces per square inch when the firing rate was one-tenth of the maximum firing rate. It is difficult to control fuel gas pressures and distribution at such low pressures. Additionally, at very low static pressures the velocity of the combustion air and fuel gas exiting the burner may become so low that the flame propagation rate exceeds the air and fuel gas velocities.
Further, in many industrial heating applications it would be very desirable to be able to utilize burners that would have turndown capabilities of greater than 7 to 1 or 10 to 1, while firing at constant air/fuel ratios. In many batch type furnaces, for example, a high firing rate is desirable to minimize heating times but very low (relative) inputs are required to prevent over-heating when the furnace and the work in the furnace reach the desired maximum temperature, while the work is "soaking". In some cases, it would be desirable to utilize burners having turndown capabilities of, perhaps, 50 to 1. A 50 to 1 turndown in conventional burners, however, would require the combustion air and fuel gas static pressures to be reduce to 50 squared, or 1/2500 of the static pressures at maximum firing rate. In the case cited above, using combustion air static pressure of 16 ounces per square inch at maximum firing rate, the combustion air static pressure at the minimum firing rate would be 0.0064 ounces per square inch.
When burner turndown requirements exceed burner capabilities in current industrial practice, one of five methods is usually used to overcome the burner limitations. The five methods are:
(a) Firing the burners on-off.
The disadvantage of this type of system is that with many processes the temperature cycling that results is undesirable or unacceptable.
(b) Increasing the air/fuel ratio as burner firing rate decreases, thus increasing the effective burner turndown.
This type of system results in greatly increased fuel costs.
(c) Modulating the burner firing rate over the range of the burner turndown achievable and then holding the combustion air flow rate constant while decreasing fuel gas flow rate.
This type of system results in increased fuel costs, although it is more efficient that the system described in b. This system also requires additional controls.
(d) Turning some burners off manually.
This not only requires operator control of burners but also results in reduced furnace uniformity.
(e) Cycling different burners on and off at a rate determined by the heat input required to the furnace.
This is a complicated and expensive system known as the "pulse" system. Aside from the cost disadvantage, many processes suffer when alternate burners are cycled on and off.